No name amp with 2 internal adjustments. What are they for?

[Lkaskel]

Hi Everyone,
So, outside of replacing pills or power cords on amps I have not had much of an opportunity to work on them. I have seen in many of the amps they may have a variable cap for alignment but I have no idea what these 2 adjustable coils are for (see attachment). It has the overall look of a Palomar amp. Any help or instruction would be great!!

Thanks!!

 

[9C1Driver]

No pic shows up, just use the attach files feature.

 

[Cable Guy]

No pic shows up, just use the attach files feature.

Yeah, it's a PDF with the pics inside. The pics show 2 variable inductor coils in the bias circuit, one for each pill input. I could be wrong but it looks like a low pass filter of sorts. Maybe has to do with the class they are biased for. Maybe to keep the pills from seeing a spectrally dirty input. Someone great will come along soon and tell you what's up, they usually do.

 

[Shadetree Mechanic]

Looks to me like they are on the negative feedback. Negative feedback is used to reduce the gain in order to increase stability. Most comp style amps don't have enough feedback in order to show higher power numbers, but this can cause the SWR to be erratic. I have never seen inductors used here, usually just the resistor and capacitor in series.

 

[Shockwave]

Those inductors are part of the negative feedback. Their purpose is to help stabilize the amplifier gain, across multiple bands of HF operation. They provide more negative feedback as frequency decreaes. This lowers gain, to offset the characteristic increase in transistor gain, accompanied with a decrease in frequency.

In this case, the slug is there to increase inductance, rather than to adjust anything. Having the slugs all the way in, makes the inductance larger and equal. They probably got a good deal on these variable inductors. I doubt they were intended to be any part of an alignment procedure. Other designs used fixed inductors.

 

[Lkaskel]

Those inductors are part of the negative feedback. Their purpose is to help stabilize the amplifier gain, across multiple bands of HF operation. They provide more negative feedback as frequency decreaes. This lowers gain, to offset the characteristic increase in transistor gain, accompanied with a decrease in frequency.

In this case, the slug is there to increase inductance, rather than to adjust anything. Having the slugs all the way in, makes the inductance larger and equal. They probably got a good deal on these variable inductors. I doubt they were intended to be any part of an alignment procedure. Other designs used fixed inductors.

Thanks Shockwave!! Is there anything I can do to be sure they are adjusted right for 10 and 11 meters? I know that is the limit of frequencies this op will be using this for. The are adjusted as they were when I opened it up.

 

[nomadradio]

The maximum-inductance setting would be your target. This will be the slug position that has the most wire wrapped around it.

This is not an adjustment in the sense that you can watch the wattage and use that as a guide. Not what it's for.

Whoever built this was probably using the cheapest coil he could get, and this was it. That's what I would do.

73

 

[Shockwave]

Thanks Shockwave!! Is there anything I can do to be sure they are adjusted right for 10 and 11 meters? I know that is the limit of frequencies this op will be using this for. The are adjusted as they were when I opened it up.

The positions they are shown in your picture, are perfect. Just leave them set where they are, there is no additional performance to extract here. Where they are set, is not as important as being set in equal positions, in order to provide symmetrical operation.